Thank you for your link. I already know it. They use the CD Changer port to communicate with the radio unit (no CD TEXT support)

For my project, I try to emulate the MP3 CD Drive. But as you suggest, I will ask on their forum some advises.

It's still not clear to me what you are trying to do.
Do you want to communicate with the CD Changer by emulating the head unit?
If so this has also already been been done here where they have emulated both the head unit and the CD Changer:
VAG CD Changer Simulator
VAG/Panasonic CD Changer Remote Control

It's still not clear to me what you are trying to do.
Do you want to communicate with the CD Changer by emulating the head unit?
If so this has also already been been done here where they have emulated both the head unit and the CD Changer:
VAG CD Changer Simulator
VAG/Panasonic CD Changer Remote Control

Hi Blues,

I never talk about CD CHANGER. As I told, I want to remove the MP3 CD DRIVE in order to replace it buy a covertion circuit which will link the PC and the unit.

I still don't understand what you are trying to do. What does "removing MP3 CD DRIVE" mean? What is the "MP3 CD DRIVE"? Is it the headunit or a seperate device connected to he headunit? OR maybe just a part inside the headunit?

How do you expect to get help when you don't post the source code or schematic?
But wouldn't it be better toask for hel in a PIC BASIC forum (if that's what you use)?

There must be a 2-way communication between the drive and the PCB. Maybe you are missing one of the SPI lines? Or maybe the data line is bi-directional?
What en generates the measure enale, cloc and data? The PCB or drive or maybe some lines aregenerated in one end and others in the orther end.